implement reading into existing buffer

include/npy.hpp

@@ -503,6 +503,43 @@ inline npy_data<Scalar> read_npy(std::istream &in) {
   return data;
 }
 
+template <typename Scalar, typename It>
+inline npy_data_ptr<Scalar> read_npy(std::istream &in, It dst_buf, size_t count) {
+  static_assert(is_same<Scalar, std::iterator_traits<It>::value_type>,
+	        "Scalar and It::value_type must be identical");
+
+  std::string header_s = read_header(in);
+
+  // parse header
+  header_t header = parse_header(header_s);
+
+  // check if the typestring matches the given one
+  const dtype_t dtype = dtype_map.at(std::type_index(typeid(Scalar)));
+
+  if (header.dtype.tie() != dtype.tie()) {
+    throw std::runtime_error("formatting error: typestrings not matching");
+  }
+
+  // compute the data size based on the shape
+  auto size = static_cast<size_t>(comp_size(header.shape));
+
+  if (size > count) {
+    throw std::runtime_error("dst_buf too small to hold file contents");
+  }
+
+  npy_data_ptr<Scalar> data;
+
+  data.shape = header.shape;
+  data.fortran_order = header.fortran_order;
+
+  data.data_ptr = &(*dst_buf);
+
+  // read the data
+  in.read(reinterpret_cast<char *>(data.data_ptr), sizeof(Scalar) * size);
+
+  return data;
+}
+
 template <typename Scalar>
 inline npy_data<Scalar> read_npy(const std::string &filename) {
   std::ifstream stream(filename, std::ifstream::binary);
@@ -513,6 +550,16 @@ inline npy_data<Scalar> read_npy(const std::string &filename) {
   return read_npy<Scalar>(stream);
 }
 
+template <typename Scalar, typename It>
+inline npy_data_ptr<Scalar> read_npy(const std::string &filename, It dst_buf, size_t count) {
+  std::ifstream stream(filename, std::ifstream::binary);
+  if (!stream) {
+    throw std::runtime_error("io error: failed to open a file.");
+  }
+
+  return read_npy<Scalar>(stream, dst_buf, count);
+}
+
 template <typename Scalar>
 inline void write_npy(std::ostream &out, const npy_data<Scalar> &data) {
   //  static_assert(has_typestring<Scalar>::value, "scalar type not